The present invention relates to a semiconductor device, particularly to prevention of dielectric breakdown of an input gate in an insulated-gate field-effect transistor device.
An example of such technology in the prior art is shown in Japanese Patent Application laid-open Pub. No. 121579/1978.
FIG. 1 shows a prior art insulated-gate field-effect semiconductor device with a scheme for preventing dielectric breakdown of the gate insulator film. The device includes an input terminal 11, an input protection resistor 12, a protection transistor 13, an input gate 14, a power supply terminal (Vcc) 15, a ground (GND) power supply terminal 16, and a transistor 17 to be protected. To simplify the description here, the input protection resistor 12 is shown to be formed of a polysilicon layer. Where the input protection resistor is formed of a doped (impurity-diffused) silicon layer, it can be regarded as a combination of a resistor component and a diode in parallel with each other.
FIG. 2 shows an arrangement for conducting human-body discharge for measuring dielectric breakdown voltage of a semiconductor device in the prior art, in which 21 denotes a power supply that is applied, 22 denotes a human-body equivalent capacitance, 23 denotes a discharge resistor, 24 denotes an object of the test, 25 denotes a terminal to be tested, 26 denotes a power supply or ground (GND) terminal, 27 denotes a switch mechanism. The human-body discharging method test is a test that simulates a situation where the human-body that is charged touches the external input terminal (the terminal to be tested) of the semiconductor device (the object of the test) causing a discharge through the input terminal into the internal circuit of the semiconductor device.
FIG. 3 shows an arrangement for conducting the charged package method test for measuring dielectric breakdown of a semiconductor device in the prior art, in which 31 denotes a power supply to be applied, 32 denotes a metal electrode, 33 denotes a testing object, 34 denotes a terminal to be measured, 35 denotes a switch means, 36 denotes an equivalent impedance for the object through which the discharge occurs, and 37 denotes a ground terminal.
The charged package method test based on the charged device model is a test that simulates a situation where electric charge accumulated on the surface of the package (insulator) of the semiconductor device due for example to friction is discharged by contacting the device pin to a conductor causing a dielectric induction current through the external input terminal into the internal circuitry of the semiconductor device.
Examples of the techniques shown in FIG. 2 and FIG. 3 are described in Japanese Patent Application laid-open Pub. No. 73375/1985.
In the past, the human-body discharge method of FIG. 2 was often adopted. But recently, it has been confirmed that the evaluation by the charged package method of FIG. 3 has a greater similarity to the phenomenon of dielectric breakdown which occurs in the market.
It is therefore urgently required to improve the dielectric breakdown voltage as determined by the charged package method.
FIG. 4 is an equivalent circuit diagram of an arrangement for implementing the breakdown test by the charged package method.
In FIG. 4, 41 denotes a power supply to be applied (corresponding to the power supply 31 in FIG. 3), 42 denotes a package capacitance that is the capacitance between the conductors within the IC package and the surface of the IC package where electric charges are accumulated. An oxide film capacitance 44 is shown that is the sum of the capacitance between the gate and the drain of the protection transistor 13 in FIG. 1 and the oxide film capacitance of the input gate 14 of the transistor 17 to be protected. An ON-resistance or punch-through resistance 45 of the protection transistor is shown which corresponds to the protection transistor 13 in FIG. 1. An input protection resistor 46 which corresponds to the input protection resistor 12 is also shown. A switch means 47 is shown which corresponds to the switch means 35 in FIG. 3. A ground terminal 48 is shown which corresponds to the ground terminal 37 in FIG. 3. A protection transistor 49 is shown which corresponds to the protection transistor 13 in FIG. 1. The response time t of the protection transistor 49 is .tau.. An equivalent impedance Z of the body through which the discharging current flow (the equivalent impedance 36 in FIG. 3) is omitted (short-circuited) from FIG. 4. This is justified where the dielectric breakdown in the semiconductor device in an automatic assembly machine is a discharge from the input or output terminal to the housing of the assembly machine.
If the switch means 47 in FIG. 4 is turned on at t=0, the voltage Vox (t) applied on the oxide film capacitance 44 is given by: ##EQU1## Here, V represent the applied voltage, Cp represent the package capacitance, Cox represents the oxide film capacitance, R represents the input protection resistance, Qo represents the charge accumulated on the oxide film having the capacitance Cox at time t=.tau., and a, b, A and B represent constants determined by the circuit constants.
In the charged package method, the breakdown occurs almost exclusively at the gate oxide film at the input gate of the transistor 17 or the protection transistor 13.
Breakdowns at oxide films are generally electric field breakdowns, which are dependent on the maximum value of the voltage Vox applied on the oxide film capacitance 44. As will be understood from the expressions (1), (2) and (3) above, the voltage Vox assumes the maximum value at time t=.tau.. The voltage at which the oxide film breaks is unchanged if the film quality and film thickness are unchanged. The voltage V.sub.B that is required to make Vox (.tau.) to be the oxide film breakdown voltage Vox b is given by: ##EQU2## Here, V.sub.B is called the breakdown voltage, and .tau. represents the response time of the protection transistor.
As was mentioned earlier, improvement for breakdown voltage as determined by the charged package method has not been studied. Rather, the study has been made to improve the breakdown voltage as determined by the human body discharge method shown in FIG. 2. In the protection circuit of FIG. 1, the breakdown occurs almost exclusively at the input protection resistor 12 in the form of fusion. To improve the breakdown voltage, it has been attempted to enlarge the input protection resistance thereby to reduce the power consumption. If the input protection resistance is enlarged, the breakdown voltage as determined by the charged package method is also improved as will be seen from equation (4). Another factor to be considered is that the gate oxide film and the field oxide film are made thinner and thinner, in line with advancement of the degree of integration of the semiconductor device. The reduction in the gate oxide film thickness leads to reduction in Voxb in equation (4) and hence reduction in the breakdown voltage as determined by the charged package method.
Effect of the reduction in the thickness of the field oxide film will be explained below:
FIG. 5 shows an equivalent circuit of a circuit used for the charged package method taking account of the capacitance due to the field oxide film. The components identical to those in FIG. 4 are shown with identical reference numerals. The field oxide film capacitance 51 whose value is denoted by Cf is increased with reduction in the thickness of the field oxide film, so that the voltage Vf (t) as applied to the field oxide film capacitance is decreased. However, the breakdown voltage of the field oxide film is reduced to a greater degree. As a result, the voltage V.sub.B that is required to cause the field oxide breakdown is lowered. If the input protection resistance 46 is increased, the response time .tau. of the protection transistor is increased. As a result, the maximum value of the voltage Vf (t) applied on the field oxide film is increased. The field oxide breakdown voltage V.sub.B is therefore reduced. Stated inversely, the breakdown voltage as determined by the charged package method for the field oxide breakdown is increased as the resistance of the input protection resistor 46 is decreased. The conventional measure of increasing the resistance of the input protection resistor 46 is in the opposite direction.
As has been described, the measure of increasing the input protection resistance in FIG. 1, leads to a larger delay in the response in the input signal and cannot be adopted in a high-speed semiconductor device. Moreover, with the larger input protection resistance, breakdown in the field oxide film can occur more easily in the charged package method.